5. THE BACKGROUND 1216 TO 1800 Å

5.1 Example of a Data Set

A very attractive set of sounding rocket diffuse ultraviolet
background radiation data at 1560 Å is provided by Onaka
(86) and is
shown in Figure 13. These data hold
considerable potential for being
an important measurement, and they are of a character that permits
useful discussion in this section.

Figure 13. Rocket measurement of the
diffuse ultraviolet background by Onaka
(86). The general
level agrees
well with the background at longer wavelengths in
Figure 11, but some
correlation with neutral hydrogen column density appears. This figure
is from (86), with
permission.

The potential of these data for being particularly important,
stems from the use of an imaging detector
(87), which means that
concerns about point-source contamination are minimized.
Figure 13 shows a background intensity of ~ 400
units, consistent with the
Johns Hopkins Aries rocket result at longer wavelengths, reported
above. With the Voyager results just reviewed the case for an origin
in redshifted hydrogen L
radiation is strengthened somewhat.

However, we also see in Figure 13 some
dependence of the
diffuse ultraviolet background on neutral hydrogen column density.
Such dependences have been reported before; the usual
interpretation is that the correlated portion of the signal is due to
the light of OB stars in the Galactic plane scattering off of dust
located above the Galactic plane. The correlation with neutral
hydrogen column density occurs because of the well-known
correlation that exists between neutral hydrogen column density and
dust (11,
111).

The theory of such dust-scattered radiation is given by Jura
(55).
A major problem with the Jura model when it is used at
moderate Galactic latitudes is its assumption of a uniform longitude
dependence in the original Galactic plane source. We have seen, in
Figures 7,
8, and
9, that that assumption is
wrong. Nevertheless,
Jura's model is useful for discussion as a first approximation.
Jura's theory has been applied by Onaka to the data of
Figure 13,
giving a (1 - g) = 0.065 ± 0.015, where a is the
albedo of the dust
grains, and g is the heuristic asymmetry factor of Henyey &
Greenstein (40).
Negative values of g correspond to predominant
backscattering. Even quite small positive values of g indicate
rather strong forward scattering.

There has long been widespread agreement, which may be
wrong, that the albedo of the grains in the far ultraviolet is high;
that a = 0.5 may even be an underestimate. This view arose from
theory and observation
(70 and reference
therein to
62). It is
supported by an ultraviolet photograph of Orion
(14) that seems to
show a bright general diffuse glow, and also by the comparison of
the Apollo 17 ultraviolet radiation field
(34) with that of
TD-1 (27)
which shows an excess that Henry
(31) attributed to
bright low
Galactic latitude diffuse Galactic light. If we do accept for the
moment that a = 0.5, Onaka's data provide g = 0.87 Å
0.03, which is very strong forward scattering.

What would Voyager have seen, if this interpretation of
Onaka's result is correct? Extinction is stronger at Voyager
wavelengths of ~ 1100 Å than it is at Onaka's 1560 Å
(105,
118). Of
course Voyager does not see the extragalactic component of ~ 380
units that Onaka's observation implies, but we have assumed that
this is because the extragalactic source does not continue below
~ 1216 Å. The question we are addressing is this: should Voyager
have seen the dust-scattered component? A simple calculation,
assuming no change in a or in g between 1560 and 1100
Å, predicts a
signal for Voyager of only ~ 90 units, which is just inside the
Voyager upper limit.

The predicted intensity will be less if the ultimate source, the
far-ultraviolet radiation field, should decline shortward of L. The
observed spectrum of Henry et al
(34)
suggests that no large decline occurs.

Next, Onaka's relation can be used (for discussion purposes) to
predict what should be seen from dust scattering at lower galactic
latitudes. This must be done with caution, as Jura's theory does not
include multiple scattering, so consider what should be seen at
galactic latitude 40°, where hydrogen column densities lie in the
range 4-10 x 1020 cm-2
(29). The prediction is
500-800 units, depending on longitude.

Finally, if indeed there are 500-800 units present at b ~ 40°
(of which 120-420 are due to dust), what should Voyager have seen
at those latitudes? The answer is, 200-700 units, depending on
longitude! No such radiation is observed
(Figure 12), and so either
this interpretation of Onaka's result (and some others described
below) is incorrect, or the grains change in their albedo and/or g
value substantially between 1560 and 1100 Å. The grains may of
course change, but the change required for, say, 300 units due to
dust at 1560 Å is rather large: at 1100 Å a (1 - g) ~
0.015, which gives
a = 0.1 for g = 0.87, and even lower a for smaller
values of g, at this wavelength.

Yet we have a very well established Voyager observation of
diffuse Galactic light (that is, starlight scattered from dust) in
Ophiuchus, described above. Voyager was fully capable of detecting
light scattered from dust, if it is there. The data from Voyager are
therefore facts of life which those who wish to ascribe much of the
ultraviolet background to dust scattering must explain.